Fix --ddp-backend=no_c10d for params that don't require grads
Fix --ddp-backend=no_c10d for params that don't require grads

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/388

Reviewed By: theweiho

Differential Revision: D13244869

fbshipit-source-id: d22c18f63f9a691ccc7245e06bc9a5b776a192b5

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq